Meniscal injuries account directly for over 50% of all surgery performed on the knee. Tears of the meniscus can occur at any age, but acute tears are most common in the 20 to 40 age group. These tears are more common in men and are often associated with sporting activities, such as tennis and football. Estimates at the incidence of acute tears vary between 24 to 60 per 100,000 population per year (Clayton & Court-Brown 2008, Arendt 1999). Medial meniscal tears occur almost twice as frequently as lateral meniscal tears (Campbell et al. 2001). Increasing age results in degeneration of the meniscal tissue with subsequent tearing. Estimates suggest that 60% of people aged over 65 have a degenerate meniscal tear. Approximately 850,000 meniscal procedures are performed each year in the United States alone (Arendt 1999).
Surgery for meniscal tears, whether acute or degenerate, most commonly involves a partial meniscectomy with resection of the torn region of the meniscus. However, this loss of meniscal tissue is now fully appreciated to increase the probability of developing degenerative changes in the joint and accelerate the degeneration in joints with pre-existing osteoarthritis (Hede et al. 1992, Andriacchi et al. 2004, Hunter et al. 2006, Roos 2005, Roos et al. 1995), therefore resulting in poor outcome (McDermott & Amis 2006). Consequently the desire is to repair or replace the meniscus to retain its function. Meniscal repair is an established procedure, but is only effective for a limited number of tears found solely in the periphery of the meniscus (Heckmann et al. 2006). Attempts to regenerate meniscal tissue using scaffolds have thus far failed to completely restore meniscal volume and are reliant on the presence of an intact peripheral rim of meniscus (Schonenfeld et al. 2007). Meniscal transplantation from cadaveric donors is an established technique (Verdonk et al. 2005) which is limited by a lack of supply, risk of infection and difficulty sizing the donor meniscus to the recipient. Clinical results at over 10 years show encouraging improvement in symptoms, and delay in progression of osteoarthritic radiographic signs but meniscal extrusion is seen in almost all cases (Van der Wal et al. 2009, Verdonk et al. 2006). While the procedure requires patients to be matched to donors, issues of sizing and compatibility will contribute to the variation in results seen (Ha et al. 2010, Lee et al. 2010).
Scaffold developers Regen Biologics and Orteq have estimated the potential value of the global meniscus market at $1.6 and $2 billion respectively based on replacing 50% of the 1.4-1.5 million partial meniscectomies estimated to be performed worldwide each year (ReGen Biologics 2009, Tan 2008).
Functional Role of the Meniscus
The menisci are two crescentic, wedge-shaped fibrocartilages lying on the tibial plateau in the medial and lateral compartments of the knee. They are attached to the underlying bone (tibia) via insertional ligaments at their anterior and posterior horns. Their primary roles are to distribute loading across the joint and to provide passive stability across the range of joint motion (Bullough et al. 1970, Seedhom 1976, Kurosawa et al. 1980, Markolf 1981, Levy 1989). Meniscal material is composed of about 75% water, about 20% type I collagen fibres and about 5% non-collagenous substances (McDevitt & Webber 1990, Wirth 1996). Therefore the tissue is naturally inhomogeneous and composite. Meniscal microstructure comprises predominantly of a dense framework of circumferentially orientated collagen fibre-bundles, with additional randomly orientated fibres tying the circumferential bundles together (Petersen & Tillman 1998). This complex arrangement means that the meniscus is locally anisotropic, providing stiffness in the circumferential direction.
Shape, structure and attachments of the menisci combine to support the primary load bearing functional role of the tissue (Masouros et al. 2008). When the femur bears down onto the tibia (so when the knee joint bears load), the knee joint is subjected to compression. The compressive force is distributed across the joint over an articulating contact area, resulting in contact stresses (contact pressure). These stresses are proportional to the load and inversely proportional to the contact area; in other words, the larger the contact area over which the load is distributed the less the contact stress. However, the geometry of the bony surfaces that articulate at the knee joint are not conforming fully, and therefore do not minimise contact stresses on the underlying articular cartilage. The menisci optimise the way the load is transferred through the joint by increasing the conformity of the articulation; as the femur bears down onto the tibia, the wedged cross-section of the menisci causes them to extrude radially out of the joint; this causes their circumference to increase. The meniscal structure resists this radial displacement by developing tension along its strong circumferentially orientated fibre-bundles (hoop stresses); this tension is then transferred through the insertional ligaments at the meniscal horns into the tibia. This load bearing mechanism occurs throughout the whole range of knee motion, as the menisci are mobile structures, able to conform to the articulating surfaces at any knee-joint position; this is precisely because the menisci are mainly attached to the tibia via insertional ligaments at their horns.
Existing Meniscal Replacements
Prior art has attempted to replace the meniscal tissue through either a scaffold system or a synthetic replacement. Scaffolds are intended to replace tissue removed during a partial menisectomy. They involve the use of a porous material which is usually sutured to the remaining meniscal tissue in the hope that tissue ingrowth will occur through the scaffold. Results have shown, however, that due to poor vascularisation in the main body of the meniscus these scaffolds are not successful in promoting regrowth; due to shrinkage, they often fail to replace the entire lost volume of tissue. Therefore, their long-term ability to reconstruct a functional meniscus remains elusive. Synthetic replacements have been attempted involving various hydrogel materials, in order to improve the general material properties of the prosthetic. However, few of these synthetic prosthetics provide evidence of effective load bearing or mechanical simulation of the meniscus.